1. Field of the Invention
The present invention relates to a high intensity focused ultrasound transducer for medical applications, the transducer generally having a shield physically attached to the transducer face.
2. Description of the Background Art
High intensity focused ultrasound (HIFU) transducers are finding increased usage in medical procedures. Similar to their cousins in diagnostic imaging, HIFU transducers share many of the same structural components. In HIFU transducers, the piezoelectric material is selected and crafted to produce the desired frequency, intensity, and total power to produce HIFU levels sufficient to lyse targeted tissue. Once a piezoelectric material has been selected and shaped, the piezoelectric material is coated with an electrically conductive material (a metallization layer) on both the front and back faces of the transducer. The piezoelectric material is ‘poled’ by applying a strong electric potential between the electrodes, activating the piezoelectric material. An electrode is connected to each metalized surface, and connected to an electrical power generator. A periodically varying potential difference is applied between the electrodes causing the piezoelectric material to vibrate longitudinally at the alternation frequency. The back transducer face generally interfaces with air or a low acoustic impedance absorber-backing; a front transducer face interfaces with the acoustic load, sometimes through an intermediate impedance matching material layer. This configuration causes an ultrasound wave front to be propagated longitudinally through the front face. Although the transducer face may be flat or shaped, in HIFU applications the front face is generally “bowl” shaped to provide spherical focusing.
In medical high intensity focused ultrasound (HIFU) applications, transducers are generally coupled to a patient using fluids. The frequency, intensity, and power used in HIFU therapy is such that reflections from the patient interface are sufficient to induce cavitation and micro-streaming of coupling agent particles (including water molecules) that can cause damage to the face of the transducer. Damage to the face of the transducer produces a number of undesirable side effects, including delamination of the matching layer from the piezoelectric ceramic, erosion of metallization on the piezoelectric material, loss of proper focus of ultrasound energy (which leads to attenuation and thermal build-up in areas that may pose a health risk to a patient), and physical destruction of the piezoelectric material used to make the transducer.
Various attempts to solve this problem have thus far proved to be unsatisfactory. In some HIFU applications, transducer shielding is sometimes found in the form of an acoustic lens placed across the transducer face. The acoustic lens provides the dual functionality of providing a degree of focusing of the ultrasound energy while simultaneously protecting the piezoelectric material from damage. Damage may come from accidental contact of the transducer face with foreign objects, or from mechanical effects of HIFU reflections in the medium used to couple the transducer to a target surface. The use of an acoustic lens has several disadvantages.
One disadvantage of this solution is that the lens also acts as a boundary layer between the transducer “stack” (piezoelectric material plus any matching layers and backing) and the target tissue. Ultrasound energy is lost through attenuation in the lens. Reflection and refraction of ultrasound energy are also problems which must be dealt with. As power and intensity increase in a HIFU transducer, the associated difficulties accompanying the use of a lens can become too great to overcome.
Thus there remains a need for a HIFU transducer that can withstand the disturbances created when the transducer is activated.
There is further a need for a HIFU transducer to be operable at extremely high operating intensities and total power levels.
There is still another need for extending the useful life of a transducer that has been damaged by disturbances.